1. Field of the Invention
This invention relates to a screen printing apparatus and method for distributing a vacuum, and more particularly, to a method and apparatus for distributing a vacuum to each of a plurality of vacuum beds arranged about the periphery of a rotatable turntable to retain articles to be printed during a multicolor, screen printing operation.
2. Description of Related Art
It is well known in the art of screen printing to print multicolored images on an article retained by a vacuum on a work carrier or platen and sequentially carrying the article to each of a plurality of print units, located peripherally about a central support, wherein each print unit prints a different color. The images printed at the several print units, when superimposed one over the other on the material, produce the desired multicolored work. It is important in multicolored apparatus of this type to retain the material to be printed at an exact and precise stationary location on the work carrier, which in the case of sheets or textiles is a flat bed or platen as the flat bed rotates to the various printing units. This is assists in the registration of the various separately colored images in relation to one another as the material undergoes its multiple printing operations, thus assuring accurately registered color imprints to form a composite image.
Earlier flat bed screen printing presses provided a single rotatable central turntable or cylinder supporting a turntable with flat beds mounted about its periphery for rotational movement with the rotating cylinder about the vertical axis of the support cylinder, wherein each bed carried a material to be printed to each of a plurality of free-standing, screen print units which were located peripherally about the turntable. That is, the screen printers were standing alone and were mounted on the floor at locations radially outward of the turntable which carried a series of circumferentially-spaced flat beds and intermittently shifted each flat bed, with its associated material mounted thereto, into a registration position beneath each of the free-standing printing units.
The material was retained in a stationary position on the flat bed by utilization of a vacuum. A single rotating central support column or cylinder supported the horizontally-mounted turntable and its flat beds. At a lower location on the rotating cylinder, a plurality of circumferentially-spaced holes were made through the wall of the support cylinder. A stationary, annular vacuum ring encircled the support cylinder about the lower holes, forming a slidable airtight seal with the rotating support cylinder. A vacuum pump was attached to the vacuum ring which induced vacuum pressure in the vacuum ring which, in turn, induced vacuum pressure in the hollow interior of the rotating support cylinder. A plurality of holes were formed in the hollow cylinder at the turntable and a plenum ring on the turntable was connected to a series of vacuum conduits leading from this turntable plenum to the vacuum beds. Reduced pressure, i.e., the vacuum was thereby transferred from the rotating plenum portion of the turntable to the underside of the flat beds, thereby providing the desired vacuum beds for securing materials thereon.
Later, as disclosed in U.S. Pat. No. 4,909,142, it was found desirable to support the multiple printing units on the central support instead of having them free-standing on the floor. The printing units are very heavy and large machines having their separate motors, printing head lift mechanisms, and squeegee and flood bar actuating mechanisms. In order to support the heavy vertical load of these printing units, two concentric vertical support shafts or cylinders were employed in these presses. An inner stationary shaft supported the printing units, and a lower cylinder or sleeve rotatably mounted on a bearing supported the turntable with its associated flat beds.
The above-described system for applying a suction plenum between an inner shaft and an outer shaft could not be used because there is a loss of vacuum pressure through the lower bearing on which the outer rotating sleeve is supported for rotational movement. The bearing extends beneath the lower sleeve, from outside the lower sleeve into the region between concentric cylinders. The bearing is not airtight but rather, allows air to pass therethrough between adjacent ball bearings. Thus, there is a loss of vacuum pressure from the region between the two concentric cylinders through the bearing. Therefore, there is a need to find means for distributing vacuum pressure in which the bearing is not required to seal the vacuum pressure.
Another shortcoming of prior art designs is that cracks in any portion of the rotating plenum portion of the turntable result in loss of vacuum pressure. The turntable supports the heavy load of the multiple, flat bed presses affixed about the periphery thereof. Since the centermost portion of the turntable is hollow there is, accordingly, a substantially reduced cross-sectional area of the turntable which must support the load. This reduced turntable thickness, combined with the rapid rotation of the turntable and the repetitive forces to the flat beds upon printing, increases the likelihood of stress cracks developing in this region. Therefore, there is a need to eliminate the requirement of a rotating, plenum portion of the turntable.
Similarly, any cracks in either of the concentric, central cylinders results in loss of vacuum pressure. Like the turntable discussed above, the central support columns are required to support heavy loads with repetitive forces imparted thereto during printing operations; thus, there is a substantial likelihood of stress or fatigue cracks developing in the columns. Therefore, there is a need to eliminate the utilization of the central support cylinders for distributing vacuum pressure.
Lastly, it has been found to be difficult to maintain an airtight seal between the stationary vacuum ring and the rotating central cylinder. Cylinders have been found not to have perfectly circular cross-sections. Such noncircularity results in gaps between the vacuum ring and the cylinder which allows the passage of air therethrough causing, a loss of vacuum pressure. Therefore, there is a need to eliminate the requirement of airtight seals with the central cylinder.
As disclosed in U.S. Pat. No. 4,909,142 which is hereby incorporated by reference as if fully reproduced herein, a Geneva stepping drive indexes the turntable and uses a driving motor, a speed reducer gear mechanism, a slotted wheel and a drive arm. All of this requires considerable space beneath the turntable. Since the column diameter is larger, a much larger vacuum box would be needed at the bottom of the column if one were to try to use the hollow space between the inner column and the outer rotating cylinder portion of the column. Thus, there was a need to eliminate such a large vacuum box and its space requirements and to eliminate the other problems, as above described, when using an annular cavity between an outer rotating cylinder and an inner stationary cylinder of a column supporting a large mass on a rotating turntable.